I’ve always thought of it more like “will slam on their breaks” than “turn into a wall”. In most states, rearending is your fault unless the person in front of you stopped faster than is normal (i.e. was hit head on by an oncoming car), and I think that sets a good standard for what you should be prepared for.

If a tree falls on the car in front of you and you hit them because they stopped instantly, it’s hardly your fault.